This invention relates to a sensing device, in particular but not exclusively for use in sensing the optical characteristics of a document moving relative to the sensing device in a transport path.
Such devices are used for the validation of banknotes.
International patent application No. WO 93/07590 describes a banknote validator in which a transverse strip on a banknote path is illuminated by light from a multi-colour transmitting station by a unitary light guide. Light reflected from a banknote is returned via the same light guide to a light receiving station. It is not possible with this banknote validator to properly sense discrete areas on the illuminated strip, since the light returning to the light receiving station is mixed across the width of the light guide. Only the optical characteristics of the banknote across its whole width are used for validation.
Another known type of banknote validator is the Armatic AL07 validator. A pair of single light guides in this arrangement deliver light from different groups of monochromatic LEDs (green, yellow, red and infra-red), and the reflected light is collected and sensed via the same light guide. The reading detectors are spaced amongst the LEDs arranged in a linear array at the distal end of each of the light guides. Again, all of the reading photo-transistors receive light reflected from all across the illuminated strip of banknote, and the optical characteristics of discrete areas of the banknote are not discernable. The arrangement is used in such a way that the spectral response received from the banknote is integrated over the whole banknote.
U.S. Pat. No. 4,922,109 describes a reading head which includes a number of reading modules. Light from a single source located externally of the array modules is delivered to illuminated strips on the transport path by light guides symmetrically arranged around the detector arrays of the sensing modules. The single while light source can be replaced by providing a monochromatic light source for each module, but still each module senses only one spectral response characteristic of a banknote.
Optical sensing apparatus suitable for validating banknotes is also described in U.S. Pat. No. 5,304,813. The apparatus consists of a linear array of photodetectors whose optical axes create a single sensor plane perpendicular to a transfer path for a banknote. The banknote is illuminated by two linear arrays of light sources located above the transfer path, whose optical axes form planes inclined to both the sensor plane and the transfer path. The light sources consist of a number of groups of sources, each group generating light of a different wavelength, arranged in a repetitive colour sequence in each of the linear source arrays. An optically diffractive element can be placed in front of each of the source arrays to effect a more uniform distribution of light intensity in the illuminated region on the transfer path. A further linear array of light sources is placed on the underside of the transfer path, arranged to lie in the sensor plane so that the sensors can be exposed to light transmitted through a banknote travelling along the transfer path.
The different groups of light sources are energised in rapid succession to illuminate a banknote passing along the transfer path with different wavelengths of light in sequence. The response of the banknote to the light of the different respective parts of the spectrum is sensed by the detector array. Because each of the photodetectors in the array receives light from a different area on the banknote, the spectral response of the different sensed parts of the banknote can be determined and compared with stored reference data to validate the banknote.
This prior art arrangement is adequate, particularly when the arrangement is used in a banknote validator in which the banknote is mechanically aligned with a lateral edge of the transport mechanism before sensing of the banknote surface. Thus, the arrangement is designed for the validation of banknotes at a limited rate.
It would be desirable to provide an alternative apparatus, in particular one which is suitable for use in the verification of banknotes at higher transport speeds, of up to 2 m/s or in that order of magnitude, and at a range of skew and offset orientations on a transport path. Such an apparatus could be utilised in a high-speed automatic teller machine.
As will be appreciated from the description which follows, the arrangement of the components of an optical sensing device and in particular the homogeneity of illumination produced thereby during sensing, is an important factor in the achievement of a practical high-speed validator in which the optical characteristics of discrete areas of a banknote are sensed.